Cause by heat scoring, and beneath the traces of green oxidation is heat blackenng. That leg specifically has been over heated over time, causing the discoloration reaction. If it was natural oxidation (like the statue of liberty, for example), then all the wires would be the same color.

Copper wire for a long time has been manufactured in such a way that it is resistant to ambient oxidation. When excessive heat is induced in the wire, that natural protection is broken down and then it is subject to turning green, but generally only where heat has damaged the wire.

Here is a basic idea of how cables are made to not oxidize when they’re just sitt ng there in normal operation.

No, the wire in the bottom terminal that’s blackened

Electrician here, that’s heat scoring. Happens when the terminal is loose, or the draw is nearing the upper limit of the installed wire for an extended period of time (looks like #8 TW, which is not ideal to run at 50a for more than 3 hours continuously, especially for how old it is). It could also be that the breaker is old enough to not be tripping reliably. Either way, there’s enough resistance with that scoring that I’ll just keep getting worse and continually building up heat until it melts the breaker and/or burns off. An electrician needs to swap that breaker and re-pull the wire (with #6s), that’s not a utility issue.